Method and apparatus for producing metallic ultrafine particles

ABSTRACT

An object of the present invention is to efficiently produce ultrafine particles having such a small diameter as 50 nm or less, a narrow range of size distribution, and a non-oxidation surface. According to the present invention, the metallic ultrafine particles are produced by dropping a raw metallic powder onto a controllably heated evaporating surface in a decompressed inert gas; instantly evaporating the raw metallic powder to form the ultrafine particle; and condensing and depositing the ultrafine particle on a trapping surface arranged above the evaporating surface. The raw metallic powder is any one of a single metal, an alloy and an intermetallic compound, preferably has an average particle diameter controlled to 500 μm or smaller so that the powder can be instantly evaporated, and is preferably supplied by a minute amount.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technology for producing metallic ultrafine particles: more specifically, relates to a method for producing the metallic ultrafine particles by dropping a raw metallic powder onto an evaporating surface from above by a minute amount, instantly evaporating it to form the ultrafine particles, and condensing and depositing them on a trapping surface arranged above the evaporating surface; and relates to an apparatus therefor. The present technology can provide such metallic ultrafine particles as to have a non-oxidation surface, a diameter of a nanometer size and a uniform particle diameter, and to be variously used as a raw material of an industrial product in many fields.

2. Description of the Related Art

As for conventional technologies for producing metallic microparticles, there are a wet method of mixing an aqueous solution containing a metallic salt with a reducing agent to precipitate metal, a dry method of sputtering a solid metal as a target with gas atoms to eject metal atoms, and a gas atomization method of atomizing molten metal into gas. However, these methods have the following problems. The wet method has a problem in that the surface of the metallic microparticles will be oxidized, because the metal microparticles are formed in the aqueous solution. The dry method has a problem of increasing the cost, because the quantity of the ultrafine particles produced per unit hour is small, thus, a long operation time of the apparatus is required in order to obtain a large amount of ultrafine particles. As for the gas atomization method, it is difficult to control the period of time for cooling molten metal, and the particles formed by the gas atomization method have a large diameter and vary widely in size. It has been difficult for any method described above to produce the metallic ultrafine particles having a small diameter (50 nm or less), a narrow range of size distribution, and a non-oxidation surface.

Another conventional technology of producing metal microparticles includes a gas evaporation method of evaporating a raw metal in a gas. For instance, Japanese Patent Laid-Open No. 4-161247 discloses a method of controlling a diameter and structure of the fine particles, by controlling the colliding condition of fine particles with gas molecules when trapping the fine particles. Specifically, the method controls a traverse speed of the particles by preparing a differential pressure between an evaporating portion and a trapping portion of a raw metal. Japanese Patent Laid-Open No. 60-78635 discloses a method that also makes use of the differential pressure between the evaporating portion and the trapping portion of the raw metal to produce the fine particles. Any method in the examples controls the particle diameter by separating the evaporating portion and the trapping portion of the raw metal, controlling and reducing the pressure in each portion, and generating the differential pressure between them. However, such a method requires an apparatus to have the evaporating portion and the trapping portion separated, which makes the whole apparatus a complicated structure. Furthermore, the method needs to increase control conditions, such as adjustment for the pressures of the two portions and the differential pressure, and has difficulty in setting the optimum condition.

Japanese Patent Laid-Open No. 9-111316 discloses a method of evaporating a raw metal and then solidifying the metal by using gas as a cooling source. However, when using the gas as the source for cooling fine particles, the fine particles may not be sufficiently cooled and has the possibility of causing aggregation during flying. Japanese Patent Laid-Open No. 2002-241811 discloses a method of making use of arc discharge as a heat source for evaporating a raw metallic material; and an apparatus having an evaporating portion and a particle-trapping portion separated. For this reason, the method has a problem that the apparatus becomes complicated.

A required condition common in all the above-described conventional methods is that a raw metal always exists as a liquid in a heat source. Specifically, those methods need to keep heating a certain amount of raw metal to a melting point of the raw metal or higher, for a long time. Accordingly, the methods need a heating device with such a large heating capacity as to keep the metal at a high temperature, and a crucible because the raw metal always exists as a liquid.

SUMMARY OF THE INVENTION

An object to be solved by the present invention is to efficiently produce ultrafine particles having such a small diameter as 50 nm or less, a narrow range of size distribution, and a non-oxidation surface.

According to the present invention, there is provided a method for producing metallic ultrafine particles comprising: dropping a raw metallic powder onto a controllably heated evaporating surface in a decompressed inert gas; instantly evaporating the raw metallic powder to form the ultrafine particles; and condensing and depositing them on a trapping surface arranged above the evaporating surface. The raw metallic powder is anyone of a single metal, an alloy and an intermetallic compound, preferably has an average particle diameter controlled to 500 μm or smaller so that the powder can be instantly evaporated, and is preferably supplied by a minute amount.

Further, according to the present invention, there is also provided an apparatus to be used in the above-described method for producing the metallic ultrafine particles. The apparatus has a vessel for forming a space of an inert gas; a heating device which is placed in a lower part in the vessel and has an evaporating surface on the top face thereof; a trapping portion of the ultrafine particles, which spreads above the heating device and is provided with cooling means; and a raw powder-feeding device for gradually dropping a raw metallic powder to the evaporating surface. The evaporating surface is a plate made from a highly corrosion-resistant material (for instance, a ceramic material), and preferably has a detachable structure from the heating device.

The trapping portion of the ultrafine particles has a structure of, for instance, permitting cooling water to pass into the portion, having a trapping surface of a hemispheric shape with its concave portion facing downward, and having a through-hole formed in the top center thereof, through which a raw metallic powder drops. The trapping surface may be a cylindrical inner face.

A method and an apparatus for producing metallic ultrafine particles according to the present invention make it possible to produce the metallic ultrafine particles having a diameter as small as 50 nm or less, a narrow range of size distribution, and a non-oxidation surface, by dropping a raw metallic powder onto an evaporating surface, instantly evaporating it to form the ultrafine particles, and condensing and depositing them on a trapping surface arranged above the evaporating surface; in addition, can prevent contamination because of not using a crucible or the like; and can save energy because of using a heat source with a small heating capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory drawing showing one embodiment of an apparatus for producing metallic ultrafine particles according to the present invention;

FIGS. 2A and 2B are explanatory drawings showing other examples on a shape of a trapping surface; and

FIGS. 3A and 3B show photographs with a transmission electron microscope of produced ultrafine particles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, metallic ultrafine particles are produced by gradually dropping a raw metallic powder onto a controllably heated evaporating surface by a minute amount, in a decompressed inert gas, instantly evaporating the raw metallic powder to form the ultrafine particles, and condensing and depositing them on a trapping surface arranged above the evaporating surface. Accordingly, the evaporating surface needs to be heat-controlled to a melting point of the raw metallic powder or higher. In addition, it is important that the raw metallic powder has a small size so as to be instantly evaporated, and for instance, has an average diameter of 500 μm or smaller. The raw metallic powder may be any one of a single metal, an alloy and an intermetallic compound.

Incidentally, a particle diameter of ultrafine particles to be produced has a tendency to depend on a distance between the vaporizing position and a trapping surface. For this reason, in order to narrow a distribution width of the particle diameter, it is desirable to drop the raw metallic particles onto an approximately constant position (for instance, the center) of an evaporating surface, and form the trapping surface into a hemispheric shape having the center adjusted to the center of the evaporating surface.

The evaporating space is controlled to a constant decompressed condition and contains an introduced inert gas, to prevent an evaporated substance from being oxidized, so that produced metallic ultrafine particles can have the surface with no oxide layer (or an extremely small amount of the oxide layer).

Embodiments

FIG. 1 shows one embodiment of an apparatus for producing metallic ultrafine particles according to the present invention. The apparatus has a vessel 10 which forms a space for an inert gas; a heating device 12 which is placed in a lower part of the vessel 10 and has an evaporating surface on the top face thereof; a trapping portion 14 of the ultrafine particles, which spreads above the heating device 12 and is provided with cooling means; and a raw powder-feeding device 16 for gradually dropping a raw metallic powder to the evaporating surface.

In this example, the vessel 10 has a structure capable of disassembling/assembling into/from three blocks of a lower vessel 10 a provided with a bottom, a middle vessel 10 b and an upper cover 10 c. The heating device 12 is installed upright in the center of the bottom of the lower vessel 10 a. The heating device 12 is a heat source for evaporating (atomizing) a raw metallic powder; may be any type of a carbon heater, a tungsten heater and an electron bombardment vacuum rapid-heating device; but needs to be capable of being heat-controlled to a constant temperature of a melting point of the raw metallic powder or higher. The top face of the heating device 12 is an evaporating surface, receives heat from a main body located in a lower part of the heating device 12, and evaporates the raw metallic powder. The evaporating surface may not be separated from the main body of the heating device, but has preferably a structure of allowing it to be separated and cleaned, in order to prevent it from being contaminated by the other raw metallic powder. For this reason, the evaporating surface 12 a in the present embodiment is made of a circular plate made from a ceramic material (such as P-BN) with adequate corrosion resistance to the supplied raw metallic powder, and the circular plate is made to be detachable from the main body of the heating device 12 b. It is recommended to form the circular plate into a diameter of about 60 to 30 mm. In addition, a thermocouple 20 for controlling a temperature is attached to the heating device 12. As a sensor for controlling temperature of the heating device 12, a radiation thermometer at high temperature is suitably used.

The trapping portion 14 of ultrafine particles is attached to the inner surface wall of the middle vessel 10 b. Here, the trapping portion 14 has a structure of allowing a coolant to pass through a coolant passage formed in the inner part (a diagonally shaded area), and having an inflow nozzle 22 and a drain nozzle 24 for cooling water connected with each other, and keeping a trapping surface at a low temperature by circulating the cooling water through the inside when the apparatus is operated. The inside face of the trapping portion 14 is the trapping surface 14 a. The trapping surface 14 a is a portion for trapping the fine particles which have been evaporated into the ultrafine particles. The trapping surface 14 a has a spherical shape, because it is important that every part of the trapping surface 14 a is apart from an evaporating surface 12 a by equal distance in order to uniformize a particle diameter; and accordingly, in the present embodiment, has a hemispheric shape with its concave portion facing downward and having the center adjusted to the center of the evaporating surface 12 a. In addition, the trapping surface has preferably a smooth surface, and accordingly is preferably mirror-finished by machining and subsequent buffing. The trapping surface 14 a has a through-hole 26 for dropping a raw metallic powder therethrough, which is formed in the center of the top part.

The raw powder-feeding device 16 is a device for dropping the raw metallic powder onto the evaporating surface 12 a by a minute amount, and is arranged above the trapping portion 14. A feeding method may be any method out of a pushing-out method with a screw or a coil, a method with the use of vibration and the like. The raw metallic powder drops onto the center of the evaporating surface 12 a through a through-hole 26 formed in the center of the top part of the trapping surface 14 a.

The upper cover 10 c has an inlet 30 of an atmosphere gas and the lower vessel 10 a has an outlet 32 of the atmosphere gas each arranged therein, so that the atmosphere and a pressure in the vessel 10 can be adjusted. The atmosphere gas is argon under a normal condition, but may be helium or nitrogen. In order to keep the atmosphere at a reduced pressure, an exhaust system (not shown) consisting of a turbo-molecular pump and a rotary pump is installed in the tip of the outlet 32 of the atmosphere gas. In addition, in order to measure and control the internal pressure inside the vessel 10, a pressure sensor 34 is incorporated therein.

When metallic particles of a raw material are dropped onto the evaporating surface 12 a of the heating device 12 placed in a decompressed inert gas, the raw metallic powders are instantly evaporated into ultrafine particles (a molten body) and fly. Thus evaporated ultrafine particles are condensed in the state and deposit on the trapping surface 14 a located above the evaporating surface 12 a. Because the trapping surface 14 a is sufficiently cooled, it can prevent the produced ultrafine particles from aggregating.

A method in the present invention employs metallic powder as a raw material; accordingly more rapidly evaporates a metal than a method of evaporating the metal from a liquid level with the use of a crucible does, such as a conventional method, because as the size of the metallic powder becomes smaller, the surface area of the metallic powder increases compared to an evaporating surface area of the metal in a crucible, though depending on the size and the quantity of the metallic powder; in other words, can increase a production rate of fine particles; can evaporate the metal in a narrow space; and can use a heat source which heats only a local part to a high temperature even without a large heating capacity, which leads to power saving and reduction in a cost. In addition, the method in the present invention makes it possible to produce ultrafine particles having the non-oxidation surface, because of producing the fine particles in an atmosphere of an inert gas with a reduced pressure; further does not cause contamination originating in a crucible, because of needing no crucible; and particularly, can produce the ultrafine particles having a uniform particle diameter, through keeping the distance between the evaporating surface and the trapping surface constant, as described above.

An embodiment shown in FIG. 1 uses a trapping surface of a hemispheric shape with its concave portion facing downward, but the trapping surface may be a cylindrical inner surface as shown in FIG. 2A. In this case, diameters of trapped metal microparticles vary depending on trapped positions thereof, with respect to the evaporating surface, on the cylindrical inner surface, so that the metal microparticles around a height position of the cylindrical inner surface corresponding to the required particle diameter are collected. Conversely, a desired ultrafine particle diameter can be secured by selecting the height position of the cylindrical inner surface. In addition, the trapping surface may be an umbrella-shaped (cone-shaped) inner surface as shown in FIG. 2B, which is an intermediate shape between the hemispherical concave surface facing downward and the cylindrical inner face. The metallic ultrafine particles can be also obtained from the above trapping surface. In this case, the obtained metallic ultrafine particles have approximating size distribution of those obtained on the hemispheric shape with its concave portion facing downward.

In the next place, a procedure for producing metallic ultrafine particles by using an apparatus according to the present invention shown in FIG. 1 will be described. First, a raw metallic powder is charged into the raw powder-feeding device 16. The raw metallic powder needs to decrease its volume and increase its heat receiving surface area, in order to be instantly evaporated. Specifically, the raw metallic powder has preferably a downsized particle diameter of 500 μm or smaller, and particularly about 100 μm. Cooling water is supplied from the inflow nozzle 22 to the trapping portion 14 of the ultrafine particles, and circulated in the inside of the trapping portion 14, and then drained from the drain nozzle 24. Thereby, the trapping surface 14 a can be kept at a low temperature while being operated. The inside of the vessel 10 is vacuumed, and then while vacuuming, an inert gas (normal argon gas) is introduced to the inside of the vessel 10 from the inlet 30 of the atmosphere gas, and the inert gas is substituted for the gas inside the vessel 10 so as to set the atmospheric pressure to a predetermined pressure necessary in production. Afterwards, the inert gas is introduced so that the atmospheric pressure is set at the predetermined pressure necessary in production. At this time, the atmosphere gas is controlled so as to enter the apparatus through the inlet 30 arranged in the upper part of the vessel 10, and to exit through the outlet 32 arranged in the lower part of the vessel 10.

When an internal pressure of the vessel 10 becomes stable, the power of the heating device 12 is turned on to heat the evaporating surface 12 a. When the temperature of the evaporating surface 12 a reaches a preset temperature, a raw metallic powder is dropped onto the evaporating surface 12 a by a minute amount, from the raw powder-feeding device 16. The raw metallic powder is dropped continuously or intermittently onto the center (specifically, within a diameter of 20 mm) of the evaporating surface.

The raw metallic powder thus dropped onto the center of the evaporating surface 12 a is instantly evaporated to form ultrafine particles. The ultrafine particles float in an inert gas and deposits on the trapping surface 14 a. Because the ultrafine particles are rapidly cooled on the trapping surface 14 a, they do not aggregate with each other, and each of them separately deposits thereon and condenses there. After the supply of the raw metallic powder has been finished, the temperature of the heating device 12 is lowered, and when the temperature has been lowered to ambient temperature, the internal pressure of the vessel 10 is returned to atmospheric pressure (by introducing an atmosphere gas therein). When the pressure inside the vessel 10 has been returned to atmospheric pressure, the apparatus is disassembled and the ultrafine particles depositing on the trapping surface 14 a for the ultrafine particle are collected.

In the next place, a test result will be described. A nickel powder (a product made by Nilaco Corporation: NI-314012 with a particle diameter of 70 μm) was used for a raw metallic powder. A temperature of the evaporating surface 12 a was raised to 1,800° C., argon gas was used as an atmosphere gas in the vessel 10, and the gas pressure was reduced to 667 Pa (5 Torr).

After the temperature of the evaporating surface 12 a and the internal pressure became stable, a raw nickel powder was gradually dropped by a minute amount onto the evaporating surface 12 a (here, 100 mm above from the evaporating surface). At this time, the amount of one drop was controlled to as very little as 0.01 g or less. After a predetermined amount of the raw nickel powder had been dropped, heating of the evaporating surface 12 a was finished, the evaporating surface 12 a was cooled to atmospheric temperature, and the inside of the vessel 10 was opened (at this time, to atmospheric air). After the vessel 10 has been opened, ultrafine particles were collected from the trapping surface 12 a.

Similarly, the temperature of an evaporating surface was set at 1,950° C., and ultrafine particles were produced.

Observation results of produced ultrafine particles, obtained with the use of a transmission electron microscope, are shown in of FIGS. 3A and 3B. FIG. 3A shows the ultrafine particles formed on an evaporating surface having had the temperature controlled to 1,800° C., and FIG. 3B shows those on the evaporating surface having had the temperature controlled to 1,950° C. From the observation result, it was confirmed that the ultrafine particles having each diameter of 50 nm or smaller could be produced.

In addition, as a result of having collected ultrafine particles produced from three different parts on a circle of a hemispheric trapping surface of which the evaporating surface had been controlled to 1,950° C., and having observed them with a transmission electron microscope, it was proved that all the particles had similar diameters (50 nm or smaller). It means that the ultrafine particles were isotropically produced with a central focus on the evaporating surface. In addition, the particle diameters-were equal in a range approximately between 10 nm and 50 nm. 

1. A method for producing metallic ultrafine particles comprising: dropping a raw metallic powder onto a controllably heated evaporating surface in a decompressed inert gas; instantly evaporating the raw metallic powder to form ultrafine particles; and condensing and depositing the ultrafine particles on a trapping surface arranged above the evaporating surface.
 2. The method for producing the metallic ultrafine particles according to claim 1, wherein the raw metallic powder is any one of a single metal, an alloy and an intermetallic compound, having an average particle diameter of 500 μm or smaller.
 3. An apparatus for producing the metallic ultrafine particles comprising: a vessel for forming a space of an inert gas; a heating device which is placed in a lower part in the vessel and has an evaporating surface on the top face thereof; a trapping portion of the ultrafine particle, which spreads above the heating device and is provided with cooling means; and a raw powder-feeding device for gradually dropping a raw metallic powder to the evaporating surface.
 4. The apparatus for producing metallic ultrafine particles according to claim 3, wherein the evaporating surface of the heating device is made from a highly corrosion-resistant material, and has a detachable structure from a main body of the heating device.
 5. The apparatus for producing the metallic ultrafine particles according to claim 3, wherein the trapping portion of the ultrafine particle has a structure that permits cooling water to pass inside the portion, and has a trapping surface of a hemispheric shape with its concave portion facing downward, and a through-hole for dropping the raw metallic powder therethrough, which is formed in the top center of the trapping surface.
 6. The apparatus for producing the metallic ultrafine particles according to claim 3, wherein the trapping portion of the ultrafine particle permits cooling water to pass into the portion and has a trapping surface having a cylindrical inner surface.
 7. The apparatus for producing the metallic ultrafine particles according to claim 4, wherein the trapping portion of the ultrafine particle has a structure that permits cooling water to pass inside the portion, and has a trapping surface of a hemispheric shape with its concave portion facing downward, and a through-hole for dropping the raw metallic powder therethrough, which is formed in the top center of the trapping surface.
 8. The apparatus for producing the metallic ultrafine particles according to claim 4, wherein the trapping portion of the ultrafine particle permits cooling water to pass into the portion and has a trapping surface having a cylindrical inner surface. 